The present invention relates to a storage material for sulfur oxides that contains a magnesium-aluminate spinel (MgO.Al2O3) and can be used as a so-called xe2x80x9csulfur trapxe2x80x9d to remove sulfur oxides from oxygen-containing exhaust gases of industrial processes. In particular, it can be used for the catalytic exhaust gas purification of internal-combustion engines to remove the sulfur oxides from the exhaust gas in order to protect the exhaust gas catalysts from sulfur poisoning.
The principal harmful substances contained in the exhaust gas from internal-combustion engines are carbon monoxide (CO), uncombusted hydrocarbons (HC), and nitrogen oxides (NOx). In addition, the exhaust gas contains small amounts of hydrogen (H2) as well as sulfur oxides (SOx) that originate from the sulfur content of the fuel and the lubricating oils of the engine. Using modern exhaust gas catalysts, a high percentage of the harmful substances, with the exception of the sulfur oxides, in stoichiometric operation of an internal-combustion engine, can be reacted into the innocuous components water, carbon dioxide and nitrogen. Catalysts developed for the exhaust gas purification of stoichiometrically-operated internal-combustion engines are termed xe2x80x9cthree-way catalysts.xe2x80x9d
Modern internal-combustion engines are increasingly operated with lean air/fuel mixtures to save fuel. While the purification of the exhaust gases of stoichiometrically-operated internal-combustion engines has reached a very high level, the purification of the exhaust gases of lean-burning internal-combustion engines still constitutes a great problem. For the major duration of their operation, these internal-combustion engines work with normalized air/fuel ratios greater than 1.3. Their exhaust gas contains about 3 to 15% by vol of oxygen. The normalized air/fuel ratio xcex designates the air/fuel ratio standardized to stoichiometric conditions.
Heavily oxidizing conditions are consequently present in the exhaust gas of lean-burning internal-combustion engines. Under these conditions, the nitric oxides in the exhaust gas can no longer be converted to innocuous nitrogen in a simple manner.
To solve this problem, so-called xe2x80x9cnitric oxide storage catalystsxe2x80x9d have inter alia been developed that oxidize the nitric oxides under lean exhaust gas conditions into nitrogen dioxide and store this in the form of nitrates. After the storage capacity of the catalyst has been reached, it is regenerated. This occurs by enriching the exhaust gas and optionally by raising the exhaust gas temperature. This decomposes the stored nitrates and releases them into the exhaust gas stream as nitrogen oxides. The released nitrogen oxides are then reduced to nitrogen at the storage catalyst with oxidation of the reductive components (hydrocarbons, carbon monoxide and hydrogen) contained in the rich exhaust gas. The storage catalyst hereby regains its original storage capacity. A storage cycle of this type lasts about 60 to 100 seconds, about 0.5 to 20 seconds being needed for the regeneration.
The mode of operation and composition of nitrogen oxides storage catalysts are known, for example, from EP 0 560 991 B1. As storage material, these catalysts contain at least one component from the group of alkali metals (potassium, sodium, lithium, cesium), the alkaline earth metals (barium, calcium) or the rare earth metals (lanthanum, yttrium). The storage catalyst contains platinum as a catalytically active element. The task of the catalytically active components is, on the one hand, to oxidize the nitrogen oxides in the exhaust gas to nitrogen dioxide under lean conditions and to reduce the released nitrogen oxides to nitrogen under rich exhaust gas conditions.
A major obstacle to the use of nitrogen oxides storage catalysts is the amount of sulfur oxides contained in the exhaust gas, since these are also oxidized at the storage catalyst under lean exhaust gas conditions and react with the storage components to form thermally very stable sulfates that cannot be destroyed during the normal regeneration of the storage catalyst. The storage capacity of the storage catalyst is thus reduced with increasing duration of operation since the storage components are blocked by sulfates.
The storage of nitrogen oxides and sulfur oxides on a storage catalyst displays pronounced temperature dependence. Storage and release of the nitrogen oxides only occur in a narrowly limited temperature interval (temperature window) that lies, for example, between about 200 and 500xc2x0 C. in the case of the frequently used alkaline earth metal oxides. The lower temperature limit is kinetically determined, whereas the upper limit temperature is given by the thermal stability of the nitrates formed. The sulfates of the alkaline earth metal oxides are only decomposed at still higher temperatures under reducing exhaust gas conditions.
To prevent the storage catalyst from being poisoned by sulfates, EP 0 582 917 A1 proposes to reduce the poisoning of the storage catalyst with sulfur by means of a sulfur trap inserted in the exhaust gas stream upstream of the storage catalyst. Alkaline metals (potassium, sodium, lithium and cesium), alkaline earth metals (barium and calcium) and rare earth metals (lanthanum, yttrium) are proposed as storage materials for the sulfur trap. Here, the sulfur trap additionally comprises platinum as catalytically active component.
It is, however, a disadvantage of the proposal of EP 0 582 917 A1 that no desulfurizing of the sulfur trap is provided. In other words, after the storage capacity of the sulfur trap has been reached, the sulfur oxides contained in the exhaust gas pass through the sulfur trap unhindered and can poison the downstream nitrogen oxide storage catalyst.
EP 0 625 633 A1 provides an improvement to this concept. According to this document, a sulfur trap is also disposed in the exhaust gas stream of the internal-combustion engine upstream of the nitrogen oxides storage catalyst. This combination of sulfur trap and nitrogen oxides storage catalyst is operated in such a manner that, under lean exhaust gas conditions, sulfur oxides are stored on the sulfur trap and the nitrogen oxides on the nitrogen oxides storage catalyst. Periodic modification of the exhaust gas conditions from lean to rich decomposes the sulfates stored on the sulfur trap to sulfur dioxide and the nitrates stored on the nitrogen oxides storage catalyst to nitrogen dioxide. Herein, however, there is a danger of sulfur dioxide and nitrogen dioxide reacting together over the nitrogen oxides storage catalyst to form sulfur trioxide and nitrogen monoxide and of sulfur trioxide being stored on the nitrogen oxides storage catalyst in the form of sulfates.
As an alternative hereto, it is possible to provide for the exhaust gas temperature to be raised in order to desulfurize the sulfur trap to values that lie above the limit temperature of the storage catalyst for the storage of the nitrogen oxides. This ensures that no stored nitrogen oxides remain on the storage catalyst during the desulfurizing of the sulfur trap. In this case the above-described reaction of sulfur dioxide with the nitrogen oxides cannot occur. This does require, however, that the sulfur oxides are only released from the sulfur trap above a specific exhaust gas temperature that, taking a possible temperature difference between the sulfur trap and the storage catalyst into account, lies above the upper limit temperature of the storage catalyst.
The requirements to be fulfilled by the storage materials for the sulfur trap during application in the processes described demand a high storage capacity, a temperature TS,DeSOx (desulfurizing temperature) for the commencement of desulfurizing that can be adapted by specific measures to the needs of the nitrogen oxides storage catalyst and the temperature conditions in the exhaust gas installation, as well as a highest possible decomposition rate for the sulfates above the desulfurizing temperature TS,DeSOx.
It is an object of the present invention to provide a storage material for sulfur oxides that substantially fulfills the requirements described above.
The above and other objects of the invention can be achieved by a storage material for sulfur oxides that contains a magnesium-aluminate spinel (MgO.Al2O3). The material is characterized in that it displays a molar ratio of magnesium oxide to aluminum oxide of over 1.1:1 and that the magnesium oxide present in stoichiometric excess is homogeneously distributed in the storage material in a highly disperse form.
U.S. Pat. No. 4,883,783 discloses the use of Mg/Al spinel to reduce sulfur dioxide emissions of catalytic cracking installations. According to this printed patent specification, the Mg/Al spinel is advantageously prepared synthetically by reacting a water-soluble, inorganic magnesium salt with a water-soluble aluminate salt. Magnesium and aluminate salt are dissolved in an aqueous medium. This causes a spinel precursor to precipitate out by neutralization of the aluminate by the acid magnesium salt. In so doing, care must be taken to ensure that the acid magnesium salt or the aluminate are not applied in excess, to prevent precipitation of excess magnesium oxide or aluminum oxide. According to the patent specification, the Mg/Al spinel still contains small amounts of at least one alkaline metal component, one calcium component, one barium component, one strontium component and one beryllium component. The material may also contain one rare earth metal component.
Contrary to the teaching of this patent specification, the storage material of the invention contains magnesium oxide in stoichiometric excess. The excess magnesium oxide is homogeneously distributed in the Mg/Al spinel and stabilizes its specific surface also when high temperatures are applied. For this purpose, a molar ratio of magnesium oxide to aluminum oxide of at least 1.1:1 is required. Molar ratios of 2:1 to 10:1 are advantageously used, in particular from 2:1 to 6:1.
The sulfur oxides are stored on the storage material of the invention substantially by reaction with the excess magnesium oxide in the form of magnesium sulfate. The Mg/Al spinel supporting structure contributes to the storage capacity to a minor extent. The storage material of the invention displays a good aging resistance, this being attributed to the fact that the sulfur oxides advantageously react with the highly dispersed magnesium oxide. This protects the high surface area support structure of the material from destruction through reaction with the sulfur oxides of the exhaust gas.
To store the sulfur oxides, these first have to be oxidized to sulfur trioxide. This can occur by means of an upstream oxidation catalyst. Advantageously, however, the storage material is itself provided with catalytically active components for the oxidation of sulfur dioxide to sulfur trioxide. Suitable for this purpose are the platinum group metals platinum, palladium and rhodium, in particular platinum that can be introduced into the material through impregnation with a soluble precursor compound. The platinum (group metals are introduced into the storage material in a concentration of up to 5% by weight, related to the total weight of the storage material.
The sulfates stored on the storage material are decomposed by lowering the normalized air/fuel ratio of the exhaust gas below 1 (enriching of the exhaust gas) and raising the exhaust gas temperature to values above about 500xc2x0 C. and desorbed in the form of sulfur oxides. This xe2x80x9cdesulfurizingxe2x80x9d restores the original storage capacity of the material.
In a preferred embodiment, the storage material is also doped with at least one of the alkaline earth metal oxides calcium oxide, strontium oxide and barium oxide in an amount from 1 to 40% by weight related to the total weight of the material. This doping call shift the temperature for the commencement of desulfurizing toward higher values. This possibility is of special importance for the combination of the sulfur trap with a downstream nitrogen oxides storage catalyst.
In addition, the storage material can contain one or several rare earth oxides, in particular cerium oxide and lanthanum oxide, that support the decomposition of the sulfates formed under rich exhaust gas conditions and at elevated temperatures. The addition of the rare earth oxides to the storage material can be 1 to 40% by weight related to the total weight of the material. Cerium oxide in combination with aluminum oxide is an unsuitable storage material for sulfur oxides (see Comparative Example 3). In combination with the storage material of the invention, it does, however, have a positive effect on the kinetics of desulfurization.
The material of the invention displays a specific surface area (measured according to DIN 66132) between 100 and 300 m2/g. Under high temperatures, this surface area decreases, but it is, however, still at least 20 m2/g after calcination at 1050xc2x0 C. for the duration of 24 hours.
The storage material of the invention is advantageously prepared by calcination of a magnesium/aluminum hydrotalcite (Mg/Al hydrotalcite). Mg/Al hydrotalcite is a twin-layered hydroxide of magnesium oxide and aluminum oxide. Its stoichiometric composition has the formula 6MgO.Al2O3.CO2.12H2O. The molar ratio of magnesium oxide to aluminum oxide is thus 6. Materials with molar ratios of 1 to 6 are commercially available. The preparation of synthetic Mg/Al hydrotalcite is, for example, described in WO 96/05140.
To convert the Mg/Al hydrotalcite into a storage material for sulfur oxides, it is calcinated at a temperature of 400 to 600xc2x0 C. for the duration of 1 to 10, advantageously 4 hours. The Mg/Al hydrotalcite used as starting material here displays the molar ratio of magnesium oxide to aluminum oxide needed for the finished storage material. Calcination transforms the hydrotalcite into stoichiometrically composed spinel (MgO.Al2O3), the magnesium oxide present in stoichiometric excess being present in ultra-fine distribution in the storage material so formed.
Calcination of the Mg/Al hydrotalcite prior to use as storage material is not absolutely essential, since the conversion of the hydrotalcite into spinel can also occur during use, for example through the hot exhaust gases of the internal-combustion engines. For a reproducible manufacture, calcination of the hydrotalcite is, however, advisable prior to use as storage material for sulfur oxides.
Before or after the calcination, the doping elements, optionally the rare earth oxides and the catalytically active components, are introduced into the storage material by impregnation with soluble precursor compounds. The material is then recalcinated for the thermal decomposition of the precursor compounds. The introduction of the additional substances into the storage material can either be carried out at the same time or successively in any order. The rare earth oxides can also be mixed with the storage material as solid substances.
There are many possibilities for using the storage material of the invention in industrial processes, such as, for example, in the fluid catalytic cracking process (FCC) and in the exhaust gas purification of motor vehicles. In the latter case, it can, as already stated, be used as a separate sulfur trap or be worked directly into the nitrogen oxides storage catalyst to be protected from poisoning by sulfur. The latter embodiment leads to a pronounced increase in the sulfur resistance of the nitrogen oxides storage catalyst and can render the use of a separate sulfur trap superfluous.
If the storage material is used as a separate sulfur trap, it is either necessary to provide an upstream oxidation catalyst to oxidize the sulfur oxides of the exhaust gas to sulfur trioxide, or the storage material itself must be provided with catalytically active components to oxidize the sulfur oxides. In this case, if the sulfur trap is disposed close behind the internal-combustion engine, it simultaneously fulfills the function of a pre-catalyst.
The storage material is advantageously used to prepare sulfur traps for purifying the exhaust gases of lean-burning internal-combustion engines. For this purpose, it is applied alone or mixed with other materials in the form of a coating on monolithic carrier bodies. These carrier bodies can be honeycomb bodies made of ceramic or metal, open-pore ceramic foams or any other gas-permeable carrier structures. The concentration of the storage material on these carrier bodies advantageously lies in the range between 50 and 200 grams per liter volume of the carrier body.
The materials that may be used in the mixture with the storage material can be high-surface area materials that are conventionally used as carrier materials for the catalytically active precious metals in the preparation of motor vehicle exhaust gas catalysts. Materials with a specific surface area of more than 10 m2/g are conventionally referred to as having a high, surface area. Suitable materials of this kind are active aluminum oxide, titanium oxide, zirconium oxide, silicon dioxide, mixed oxides hereof zeolites or physical mixtures of these oxides. The storage material here can be in a weight ratio of 1:5 to 20:1 to the additional substances.
The catalytically active components (platinum, palladium, rhodium, ruthenium, iridium osmium) are introduced into the sulfur trap by impregnation. The sulfur trap can further be provided with promoters from the group of the transition metal oxides. Suitable transition metals that support the catalytic function of the sulfur trap are zinc, nickel, chromium, cobalt, copper and silver.
Thus, the storage material can be used in sulfur traps in combination with several other components. A preferred sulfur trap includes, for example, a mixture of aluminum oxide and the material of the invention. In addition, it is also possible to dope the storage material with alkaline earth elements. In principle, these additional components of the sulfur trap are also able to bind sulfur trioxide in the form of sulfates.
If, for example, aluminum oxide is used as the sole material, the sulfur components are bound in the form of aluminum sulfate. However, this greatly reduces the specific surface area of the aluminum oxide. As a consequence hereof, the rate of formation of aluminum sulfate diminishes with increasing aging of the sulfur trap. By mixing the aluminum oxide with the storage material of the invention, this aging process can be largely avoided, since in this case the more stable magnesium sulfate is advantageously formed.
In the following examples and comparative examples, various formulations of sulfur traps with and without use of the storage material of the invention are compared with one another. To ensure the comparability of the various formulations, the amounts of the individual components were in each case adjusted in such a way that the theoretical total storage capacity of the sulfur traps was about 4.7 mol sulfate in all the examples. In this context, it was assumed that aluminum oxide can be completely converted into aluminum sulfate and magnesium oxide completely into magnesium sulfate. This approach was also adopted in the case of the doping elements. In each case, the amount of the doping elements was adjusted so that their theoretical storage capacity was 0.17 mol sulfur per liter of the sulfur traps. The storage capacity of the spinel was calculated as the sum of the storage capacities of the ratios of magnesium oxide and aluminum oxide contained therein.
Platinum was used as the precious metal component in all the sulfur traps.